Conveyorized Tunnel Furnaces or Kilns

It is wise to return a conveyor within the furnace to save heat loss and to prolong its life by minimizing the amplitude and the frequency of the temperature cycle to which the conveyor materials are exposed. Many materials last longer if kept hot, rather than being constantly cycled between hot and cold. For flexibility during production line delays, it is advisable to provide a temporary storage area at each end of a conveyor furnace. 

A common problem with many continuous furnaces is an accordion effect that occurs after line stoppages. Continuous furnaces are wonderful as long as they maintain steady-state operation. To envision the accordian effect, think of the changes with passage of time of the temperature pattern throughout the length of a fumaoe with temperature sensors located at the traditional positions near the ceiling of the furnace and near the load-exit-end of each zone. 

To prevent this problem, all control sensors should be close to the level of the tops of the loads. Input control sensors should be within about one-fourth of their zone length from the load entry end of their zones. Over-temperature sensors should be 5 to 10% of their zone length from the exit end of their zones, and set at the maximum furnace temperature allowed. With such a sensor-positioning arrangement, a modem quick-recovery temperature control has a chance to avoid a heat delay following a mill delay. 

Tunnel kilns, widely used in firing ceramics and carbon shapes, use a long train of cars as a conveyor Each car may be similar to, but often narrower than, the car of a batch-type car-hearth furnace. Much of what is discussed in this book can apply to ceramic kilns, but the ceramic industries have so many publications on kiln construction and operation that this text will not dwell on them specifically. 

The heating capacity of furnaces in this midtemperature range can be determined by calculating heating curves, as discussed in sections 4.6 and 8.2. The lower radiation intensity in this range warrants more attention to convection, surface exposure, and circulation (chap. 2 and 7). 

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Conveyorized tunnel furnaces or kilns may be stretched versions of their batch equivalents, divided into several zones. Many types of conveyors are used. Figure 4.6 shows... 

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